Sterndrive Losses? Less Than You Think!

Tremendous effort goes into good boat set-up. My conversations about set-up too often turn to power consumed by a drive train,or generated by an engine, and always… propellers. The goal is efficiency – accepting some sacrifice for boat control. Really, your goal is the euphoric joy and adrenalin rush of high performance boating! My goal is to help you get there.

Mercury Racing’s test rig for measuring parasitic losses in drives and transmissions.

Here, I’ll focus on drives. (We’ll cover engine power and props later.) Between engine crankshaft (drive input) and prop shaft (drive output), basic functions are required: gear reduction (so props are efficient); offset of input vs. output shafts (so they’re wet) and ability to change direction (steering and trim are good).

Mercury Racing employs several sterndrives for those functions. Each occupies a unique performance envelope and capacity. Unfortunately, each has parasitic losses: clutch slippage; gear efficiency; number and nature of gear interfaces; U-joint friction; bearing drag; gear oil (quantity, temperature, viscosity and lubricity); and oil windage/pumping losses. So here, drive by drive, are the results of those parasites…

The Bravo XR with optional Integrated Transom System (ITS).

We package the Bravo One XR drive with our 525 EFI and 600 SCi engines. XR has an internal cone clutch transmission with positive engagement: No slip. It has two forged, straight bevel gear sets. The lower gears run wet; uppers are oiled by a groove in the vertical shaft. At 5,200 rpm input, a Bravo One XR consumes 25 hp or 4% of a 600 SCi’s output.

CAD image of a dry-sump NXT1 sterndrive.

The , NXT1 with an external cone clutch transmission, is one tough, efficient machine. This drive/clutch combination was developed for EU662 SCi and 700 SCi engines. Both transmission and drive are dry-sump designs. Drive gears are forged straight bevel. Like the Bravo, this transmission is a positive engagement design. There is a noticeable efficiency gain using a cone clutch, dry-sump oiling and following lubricant recommendations: At 5,200 rpm input, an NXT1drive consumes 10 hp. Its transmission takes 5 hp; however, at Racing we dyno engines after the transmission (662SCi and up). Therefore, only 10 hp is the loss to the prop, or 1.4% of a 700 SCi’s rated power.

This is a section of Racing’s dry-sump Six drive — the most successful racing drive ever produced.

Our engineers package the dry-sump Six drive with 850 SCi, 1025 SCi, 1075 SCi and 1200 SCi engines. It employs three forged, straight bevel gear sets, two spur “splitter” gears and twin pinions to reduce its hydrodynamic profile.  At 6,000 rpm input, a dry-sump Six consumes 12 hp. The hydraulic transmission consumes 15 hp. Only 12 hp is the loss to the prop, or 1.0% of a 1200 SCi.

Here’s the new M8 highlighting the shorter input-to-output shaft spacing.

The dry-sump M8 employs a dry-sump hydraulic transmission, too. Both were designed for our 1350 engine. It uses a similar gear configuration and oiling approach as the Six – only scaled up for the Big Fat Monster Torque of QC4v. The transmission has 35% more friction surface than a Six transmission. At 6,500 rpm input, an M8 consumes 22 hp and its transmission consumes 17 hp. But, only 22 hp is the loss to the prop, or 1.6% with a 1350. While that is 10 hp more than a Six, it’s almost entirely due to higher rpm with bigger gears — the price of higher power capacity and longer life.

In pursuit of performance, I believe there is more to gain from set up, prop selection and drive height optimization than by tweaking or over-powering drives: From M8 to Bravo, drive parasitic loss ranges from only 1.0 to 4.0 percent. And losses go up exponentially, not linearly, with input speed. So, choose the right power package for your fun, run it at no more than rated maximum power and rpm and please use our recommended lubricants, properly filled (both very important, more later). You’ll be rewarded with top performance and robustness.

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42 thoughts on “Sterndrive Losses? Less Than You Think!”

    1. You’re making me dig through dusty memory banks, Stefan. We have not built them for over 15 years. I believe the wet sump six was 55 to 70 hp loss, depending on the input rpm, gear lube and oil fill level. When we designed that drive back in 1986, before my company was part of Mercury, we did not have the tools to measure accurately. We were thankful to have a drive that could handle the horsepower. When we finally had the tools, we were disappointed to learn that the losses were higher than we expected. We then developed the dry sump six.

  1. I have a project on a 30′ Spectre cat with 525 Bravo XR 1.35.1 drive that is set -1.5 up, it runs 118. I want a pair 700 Mercs with NXT drives, What x dimension would you recommend?
    Thank you.
    Fernando Gale

    1. Mr. Gale, let me check with our technical support managers after the Holiday to get their recommendation. Personally, I don’t recall NXT drive set up for a 30 Spectre.

      I suspect the prop shaft center line would be about an inch higher than with the Bravo (presuming the Bravo was correct — and from the speed, it probably is). That’s because the NXT cleaver props are larger diameter; moreover, the NXT places the prop farther back than the Bravo. Brad or Mike will know for sure. Check back for an update Jan 4 (or sooner if I can reach them). Thank you for your interest.

      Happy Holidays,
      Fred

      1. Well, Mr. Gale, I was too conservative.

        From Mike @ 7:45 pm on Christmas:
        “Besides our own Spectre Cat [one of our test boats], we have not done one of these. Starting X dimension: 3.5″ above bottom. Props turning inward with a diameter of 16.75″ 18 rake. Pitch depends on gear ratio. 1.35 gear ratio for best overall acceleration and top MPH. 37 to 39 pitch prop. This would be a potent combination.”
        And if Mike is too aggressive on the x dimension, one can always space the prop shaft down. Good place to start.

        From Brad @ 8:44 am on Dec 27, a very important consideration:
        “Based on previous installs similar to this, I would recommend 3” above to allow for adjustments [about the same as MIke recommends]. Being familiar with the Spectre cat, I do not think they have enough flat transom to allow this in the boat’s current form. …the Bravo x dimension is going to be too low unless [you] build up the transom portion of the deck to allow a higher x. The higher X is very important with the NXT1. If installed too low, it will make the boat run on its nose… Brad”

        I would strongly recommend a look at the transom to see if there is enough space to move up and also, check the deck to be sure there is enough space to clear the 700SCi screw charger and headers. You may end up modifying the hatch — or more involved, the rear deck, transom and hatch. If you like, give Kevin Skiba a call on Jan 4. He can send you an installation template for the NXT transom. You can reach Kevin at 920-921-5330. Best of luck.

  2. I have a 1996 fountain fever with 502mpi and bravo one drive, which runs 65-67mph. I want to start upgrading for more topspeed but i am very confussed from where to start. some friends of mine tell me to start from the engine- others drive, prop, lower unit, extension box etc….
    what is your opinion please?
    Regards,
    Owen.

    1. I’m assuming yours is a 27 Fever with a single 502. It depends upon your appetite: you looking for two or ten mph? Here’s my opinion on your options:

      1) With an existing boat, the most cost effective upgrade is usually your prop. That can be a crap shoot because it depends on how well the boat was set up initially and how well the prop was maintained. Reggie was pretty good at Vee bottom setup. You might see a mile per hour or three — or nothing. Start with a stock Maximus prop. Lab finish if you need to. Talk to Scott Reichow in Racing’s prop department: 920-924-2037.

      2) Next place to look is the engine, but the price tag can go up fast. One cost effective way is to purchase a 525EFI remanufactured engine and some headers. If you go that route, plan on upgrading to a Bravo XR, too, just to stay reliable with added power.

      3) Sell your boat and buy one that already has a 525EFI/Bravo XR package. The work will be done and somebody else will have taken the depreciation.

      Without an engine upgrade, a different drive will yield nothing. The Bravo you have is very efficient and suitable to the current engine. On a boat this size, an extension box is usually a bad idea as they change the weight distribution. Modifying a 502MPI? Question is: Where to start; where to stop? There are lots of shops that would love to take your money, but I would not even consider it. See 2, above — the 525 already has heavy duty rods, forged pistons and a more robust valve train and has been extensively tested for your application. Ask yourself: “Do I want to go boating or do I want to go boat fixing?”

  3. I have a 1996 Campion 910 Chase, it’s 30′, has twin EFI 502 engines. It also has bravo three drives on it. I’m thinking these aren’t the best choice for top speed potential, although it does top out at 85mph or slightly better on a perfect day. I think some Bravo one’s is what it needs. What’s your opinion on these? Thanks

    1. With twins, you’re right. The advantage of Bravo threes is their mid-range and lower speed efficiency – especially with a full load or heavier boat. Your Campion is not that heavy. That said, your boat is running well with the power you have. So, I have to ask, what are you hoping for? If you pick up a few mph top end (and potentially lose one or two at mid-range), is that really going to light your candle? If it were my boat, I’d leave well enough alone — or trade up to a new stepped bottom boat with 565s or newer stepped bottom with 525s.

      1. I was thinking that Bravo threes have a quite a bit more drag, and so I was hoping swapping them out might gain me more than a few mph on top end. Also the ss props are frosted, is this a sign of cavitation?

        1. Actually, the frontal drag of Bravo Threes is about the same as Bravo Ones. When you back off quickly, there is more drag due to the extra prop blade area and that’s usually what you want (or you wouldn’t have backed off quickly).

          “Frosted” blades could be light corrosion, cavitation or ventilation. I can’t say without seeing the blades. If it’s all over the blades and hubs, it is most likely corrosion. Check the condition of your drives’ sacrificial anodes and bonding straps (little cable jumpers). They should be present and not painted; pitting of the anodes show they’re doing their job. If they’re almost gone, replace them. Have a mechanic check the function of merCathode if you have that system installed.

          If frosting’s localized, oval shaped (perhaps 1/4 inch by 1 inch in size), and present on all the blades of the same prop at about the same distance out from the hub, then it’s likely cavitation burn. If it’s just on the rear propeller, it may be induced by the front propeller. That can come from dings in the blades, upstream interruptions (speedometer or water pickups) or air sucked from the surface.

    1. There is an electronically actuated hydraulic transmission between the engine and the M8 drive.

  4. I’m setting up an older (1998} 36 Skater with 1075’s. It had 900sc with 4 blade props, I’m switching to 5 blade props. Initially x was 4″ and 40 pitch props. Boat had high slip numbers and porpoised a lot.

    1. David, are you looking for confirmation of your prop direction – or a recommendation on a new x dimension and prop? Either way, 1) what rpm were the 900s turning with 40 pitch 4 blades, 2) what diameter were they, 3) who was the prop manufacturer and 4) are you running Mercury Racing #6 drives? Thanks.

  5. I currently have a setup of a Bravo IX drive with ITS on a 29 stepped RIB with Yanmar diesel. The prop I use is a Bravo I – 29 Lab.
    I have noticed that the same boat with the same prop without the ITS seems to rev faster and higher.
    Can an ITS affect boat performance so much? How much parasitic drag does it have if any? or is the length of the boat too short, to take advantage of the ITS?

    Last if I were to upgrade to an XR drive what would be the loss in performance from my IX drive?

    1. Puzzling. There are often differences boat-to-boat — even among the same make and model. There can be differences between two props marked with the “same pitch.” There can be differences in boat balance and water flow across the bottom. There can be water pickups or other installation variances that disturb that flow.

      There is no drag associated with ITS. A little above idle, it is mostly out of the water, if installed properly. At planing speed, it is completely out of the water. ITS’s primary advantage is packaging and rigidness of the hydraulic steering which translates to improved boat control. There is also an advantage on most boats of additional drive set-back. Not all boats benefit from that; it’s about balance and cleaner water to the prop. Usually, with a stepped hull, the ITS setback helps get non-aerated flow to the prop. Sometimes on a shorter hull, the setback can move the balance too far aft. Hard to diagnose cause without first hand observation.

      You should see no difference in performance between a Bravo One X and a Bravo XR. The XR components are more robust, but the small change in internal drive efficiency is so small that you’ll never see it in your boat.

      1. Well the primary reason that I choose the ITS was the steering and the rigidness since I use the RIB in the Aegean sea, where we often encounter high winds and extremely choppy conditions.
        For your reference the boat is a 2009 Goldfish 29 from Norway and it has won the Round the Britain race in 2008 (under 30 ft) with the same setup.
        I fully understand the non-aerated theory and maybe the other Rib sucks dirtier water and the prop overspins. You have confirmed some questions that I had, and I think it must be down to boat balance and probably as you said moving it too far aft. I will experiment with weight distribution and maybe with a 5 blade prop. Do you think that there could be some gains by going to a Maximus.

        1. Panos,

          I need some additional information to answer your question regarding the performance of a Maximus prop on your rig. Please respond with your drive gear ratio, top speed and wide open throttle engine operating RPM. Those numbers, plugged into the prop slip calculator, will determine if you should stick with the 4-blade or consider the 5-blade Maximus.

          Scott

          1. Drive gear ratio is 1.36:1. Top speed is around 54-55 knots at 3800-3900 rpms. The motor is capable of turning 4200 rpms. According to my calculations, slip numbers are between 19-20%. I currently run a 29 Bravo I Lab. For sure I need to at least go up 200 revs.

            Thanks Panos

          2. Panos,

            The four blade Bravo I may be the best choice for top-end speed, even with your slip at 19%. Finding the right balance between slip, pitch and propeller handling while maintaining the appropriate engine RPM is a challenge. Let’s look at some options and what you might expect for performance between the Maximus and stock Bravo I propellers. To get 3850 RPM with a stock Maximus, you would have to step down in pitch to possibly a 15-5/8″ x 24″. The five blade would reduce slip but your top-end speed would drop significantly as well. The 15″ x 27″ Lab Finished Maximus ST (shortened tube) would deliver similar 3850 RPM and your slip would decline to the 12 to 14% range. The lower slip is good and if the boat reacts well with the five blade, you could expect speeds in the 62.2 to 63.4 mph range. The stock 28″ pitch Bravo I will drop your RPM by 150 down to 3700 RPM, with an estimated top speed of 62 mph. If you drop to a 26″ pitch Bravo, the engine RPM typically picks up 150 RPM moving the tachometer to 4000 which could deliver 62.5 to 63 mph speeds.

            Have fun testing.

            Scott

  6. Hello Fred ,i have a old 1995 36 skater cat ,was a race boat couple years back. I have 2 525 efi’s and a older set of ssm #6 drives wet sump drives . What prop would you recommend for my set up?thanks so much for your time.

    1. James,
      Usually, this set up would run a 16.50″ diameter. Pitch size depends on the gear ratio. Give Mercury dealer Bob Teague in Valencia, California a call (661-295-7000). Request to speak with Bob. He has several propellers in stock with several pitch options specifically for the 525 EFI – 6 drive application.
      Scott

  7. hello Fred,

    I m building a 30 Doughwright designed Predator cat and looking for the best x dimmension and how much prop blade have to turn into the tunnel .
    I have a set bravo xr ITS sportmaster drives 1:50 standard lenght as well as a set -2″ lowers whit 1:35 gears .we are gonna run 500 hp ls based engines that gonna turn 6000 rpm .
    we are thinking to start at 3.5″ above the hull whit propshaft and then space down but not have a clue how far the props need to turn in the tunnel

    thx armand

    1. Hello Armand.

      I will forward your question to our Product Integration Manager. We will reply to you shortly.

      Rick

  8. I’m not sure if this will work
    I’m restoring a Star Craft 2010LX from the stringers up. So I’m planning on upgrading from a 5.7/alpha (250hp) C=
    to a 454/Bravo (500hp) with a extension box in an attempt to gain 20mph. I’m wondering if this is the right direction?

      1. I’m not sure the stock engine was 250 hp
        Maybe 300hp?
        I redoing stingers & bulkheads because of rotten wood ,so I’ll be adding strength by adding some cross brasing& gassing everything in real will unlike the factory job

  9. I’m also considering just going with a 383 stroker-450 hp for 3 reason I can use old brackets & hardware, it’s lighter, an possibly be more cost efficient.
    But I’m still not sure if a existential box will work on such a short boat

  10. Hi, I recently repowered my 99 Fastech 353 replacing the carb HP 500 motors with 548s. We have factory standoff boxes and the std lower Bravo 1.5 propshafts are set at 4.5 inches below the running surface. The boat runs with a bow down attitude using the stock 30p bravo 4 props which seems to be eorse dine instslling the new power. Mirage 3’s didn’t help raise bow much and slip a lot more. Using 28 or 30 bravo 4’s we only picked up 3-4 mph with new motors despite a HP improvement on the dyno of 126 (154 corrected) per motor. I do not have planing issues and really never need the tabs. The new motors max tq and hp is about 1000 rpm higher than the old motors so we’re now at 4800/5800. Where would you go with this setup to raise the bow and find the speed improvement we expected but are lacking? Thank you.

    1. Joe,
      What was the original performance data, i.e. style of props (Lab or stock), the pitch, engine rpm and GPS speed at wide open throttle? Were the stand-off boxes and drive height the same on the original set up? What were the running characteristics? Did the bow rise low from the start?
      Scott

  11. Hi Scott, As far as I know the boat originally came with stock 30p bravo props. Thats how I bought it with the original carb hp 500s and it ran 79 mph at 5100 rpm. We are now only running 82 at 5800 rpm (which is max hp range for new motors – we dynoed 650 hp/695 chp) with a total of 250+ more hp over the original mills. The boxes, tabs and drives are factory original and aside from the motors nothing has been altered. The bow down running attitude was there when I got the boat and I hear it’s a common trait with the 353’s but in this case I think we need more air under thd hull to free it up and get speed. The bow seems to be getting pushed down more as more power is added. Is 4.5″ propshaft height too low or will rising help speed increase? Do we simply need labbed props or another 200 hp from thd motors? What else could be holding us back? Thanks

    1. Joe,
      Usually Formula ran our Lab Finished Bravo I. If they have a polished finish, they are standard. I they are a satin finish ((back then to 2014) they’re Lab Finished. Looking at your information, (30″ pitch, 1.5 gear ratio, 5100 rpm and 79 mph speed) results with 18.2% slip. This sounds about right for Lab Finished Bravo I props on a step hull. With a change in horsepower, spinning the same 30″ pitch props 5800 rpm and now reaching speeds of 82 mph enhances prop slip to 25.4%. What’s the cause of the added slip? If nothing has changed in drive height, gear ratio, props (no damaged) and you just increased horsepower – we should get the same 18.2% slip. With the engines getting bumped up to 650 HP (from a conservative view) I would expect to see 87 mph running the same 30″ pitch props at 5600 rpm. Any idea why the slip is so high?
      Scott

  12. Scott,
    Nothing has changed with the boat except the motor replacement and trying various props. It seems that no matter what we try on the new motors we cant get past 82. Lower pitch and less blades raise rpm but not speed. One thing may be motors are not making 650 at 5600 for some reason. Some people I have spoken to have suggested we have reached terminal velocity but I can’t believe that this hull can’t go any faster. Unfortunately we are out of the water for the season but my plan for over the winter is to do some cylinder head upgrades dyno again and get the 30s labbed. Let me know your thoughts about the prop shaft height and if you think we would benefit from shorty lowers. Thanks

    1. Joe,
      While you’re putting up with winter, here’s something for you to consider. In 2005 we tested with Formula (a pair of 600 SCi’s coupled to Bravo I DWP (Dual Water Pickup) sterndrives in a 353 FasTECH). The prop shaft was 4.125″ below bottom, with the top 3 water inlets plugged. We ran Racing’s Lab Finished Maximus (15.25” x 31P 5-blades). We had (3) people on board with 200 pounds of added weight, 43 gallons of gas, with an air temperature of 71 degrees. We reached top speeds of 92.5 mph at 5250 rpm (glass water conditions) and 95.2 mph with slight ripple on the water.
      All the best,
      Scott

      1. Scott,
        Thanks for the info. So it sounds like we are fine with the existing propshaft height. I’m not sure what the dual water pickups and blocked plugs would contribute to speed but I would like to know what a 600sci dynos at (base and corrected hp) before hooking up drivetrain. I have been avoiding the 5 blade props to limit stress on the stock 1.50 Bravo 1 drives plus we are trying to get the nose up and normally adding blades raises the stern. I was thinking of just labbing the 30P Bravos but we could just go to 5 blades if you think that would be better since I do not abuse the drives as a throttle man.
        Thanks,
        Joe

  13. Hello , i have a 30 dough wright designed cat and have a pair bravo xr it’s sportmaster drives that I gonna be using I have now measured that when I mount the it’s transom high as possible the propshaft is even whit the bottom so whit -2″ lowers I can get it up to 2″ above running surface would this be enough whit getting a bit that now steer ?
    The boat has a 16 ” notch so I’m afraid the water will not go underneath the bullet of the lower unit as it go s up behind the boat

    Thank you

    Armand

    1. Good morning, thinking about repowering a 1986 Baja sport 220 I had since new just had hull redone @ Baja, interior new,but still a60mph boat. Want to change to a 383 striker & have to go to a bravo drive. Will a bravo Xr sport lower, fit a regular bravo upper. Want to try and reduce drag. As I have read about with my boat. The alpha SS is no longer available. Any input is appreciated have been a loyal merc. Guy since I started boating over 40 years ago. A new boat is not an option I live on a lake. Besides looking at A formula. PC for my condo in Islamorada. Thanks again for any help. Joe

      1. Joe:
        Yes. The Bravo XR Sport Master (Std and Short) or Sport (dual water pickup crescent leading edge gear housing with short strut) will fit the standard Bravo One housing.

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